A telecommunications network may employ a combination of several approaches for restoring traffic flow interrupted by equipment failure or malfunction. For example, a network may rely upon a 1:1 automatic protect switching for some portions, distributed restoration (DRA) for other portions or domains of the network, and finally a centralized control of rerouting for situations not readily handled by the other approaches or schemes.
The automatic protection switching (APS) scheme, although wasteful of bandwidth, nonetheless is useful in some cases where simplicity and speed are required. The DRA scheme, on the other hand, takes slightly longer and may conceivably be considered more complex. But the DRA scheme is also more adaptable than APS and can leverage mesh restorability. For both APS and DRA schemes, restoration is triggered by the detection of transmission alarm conditions that result from occurred failures.
In a network where there are a number of schemes such as APS and DRA, even though the failure may quickly be circumvented by the APS scheme, the DRA provisioned nodes of the network may also begin to respond to the same failure, all of the while oblivious to the activities being carried on by the APS process. Such unnecessary triggering of the DRA process is undesirable, insofar as the DRA provisioned portion of the network, while the DRA process is ongoing, becomes vulnerable as the DRA process seeks to reserve spare links for the distributed restoration. Accordingly, any subsequent real failures could be precluded from complete and timely restoration due to this false triggering of the DRA process. To overcome this shortcoming, the DRA scheme must verify the genuineness of the failure before taking action.
It would appear that a simple solution is the provision of a timer. However, the provision of a timer for such multi-restoration schemes is inadequate because there are numerous links that interconnect the nodes of the network, and any one of those links can generate an alarm condition. It is therefore possible for two or more separate short duration alarm events to overlap enough so that those events would appear to be a single long duration alarm event, thereby activating the DRA process.
There is therefore a need for a robust method for determining when a DRA process may be activated.
Moreover, there is a need to have the DRA process start as quickly as possible, when it is needed, and yet not be triggered by false alarms.